Watermarking is a well-developed art, with a great variety of techniques. Generally, all vary an original signal (corresponding, e.g., to audio or image data--video being considered a form of image data) so as to encode auxiliary data without apparent alteration of the original signal. Upon computer analysis, however, the auxiliary data can be discerned and read. (For expository convenience, the following discussion focuses on image data, although the same techniques are generally applicable across all watermarking applications.)
A problem inherent in all watermarking techniques is the effect of the underlying image signal. In this context the underlying image signal--although the intended signal for human perception--acts as noise for purposes of decoding of the watermark signal. In most cases, the energy of the image signal far exceeds that of the watermark signal, making watermark detection an exercise in digging out a weak signal amidst a much stronger signal. If the encoded image has been degraded, e.g., by scanning/printing, or lossy compression/decompression, the process becomes still more difficult. As watermarks become increasingly prevalent (e.g., for device control, such as anti-duplication features in reproduction systems), the importance of this problem escalates.